As the optical communication industry journeys toward a more advanced version of the next generation passive optical network standard (e.g. NGPON2) which requires a 40 gigabits per second transmission rate over 100 kilometers (km) of signal transmission over fiber, the performance of conventional optical transmission systems such as ones which utilize an Intensity-Modulation Direct Detection (IMDD) technique may fall short of the required standard even though an IMDD based device could be inexpensive and simple to design. Also since the conventional On-OFF keying (OOK) modulation technique is bandwidth inefficient, the Orthogonal Frequency-Division Multiplexing (OFDM) modulation technique which is more bandwidth efficient in comparison to the OOK modulation technique may become an enhancement.
The conventional IMDD based system may be restricted by radio frequency (RF) power fading. More specifically, when a signal, for example, an electromagnetic (EM) signal, is amplitude or intensity modulated, normally a double sideband is formed in the frequency domain. As the modulated EM signal traverses through the fiber, the modulated EM signal experiences from the fiber a dispersive effect which causes each side of the double sideband to phase rotate in the opposite direction. After the EM signal is received by an optical receiver, the RF power fading occurs in the received EM signal due to the effect of the destructive/non-constructive interference of the phase rotation of each of the double sidebands. As the EM signal propagates down the fiber, the worsening effect of the RF power fading becomes more and more apparent. Therefore, a solution which involves an optical single sideband modulation technique has been proposed.
Supposedly, if an EM signal only contains a single sideband, the problem of RF fading would not ensue. However, the means to achieve the single sideband modulation may require the Mach-Zehnder modulator (MZM) which could be twenty times more expensive than the direct modulation laser (DML). Also, the modulation method of the MZM modulator is amplitude modulation. After a modulated EM wave is received by an optical receiver, signal to signal beat interference (SSBI) with the same bandwidth as the single side band signal may occur in the lower frequency spectrum of the EM signal. Therefore, in order to accommodate this type of system, a guard band is usually needed which would then result in twice the transmission bandwidth. Based on this reason, an improvement based on the conventional IMDD technique to realize the required high speed transmission for the next generation passive optical network (NGPON) may still be feasible.